The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
An internal combustion engine (ICE) combusts an air/fuel mixture to produce drive torque for a vehicle. Combustion byproducts are exhausted from the vehicle via an exhaust system. Exhaust gas from the exhaust system may be received by a turbocharger system. The turbocharger system increases torque output of the ICE by increasing air flow to the ICE. The turbocharger system compresses and increases flow of inlet air based on flow rate of the exhaust gas. The compressed inlet air is drawn into cylinders of the ICE via an intake manifold for combustion purposes.
A turbocharger system may be a single-stage or multi-stage system. A single-stage system may include a single turbine that increases boost pressure in an intake manifold. A multi-stage system may include a primary turbine and a secondary turbine that are arranged in series. The primary turbine and the secondary turbine receive exhaust gas and provide respective compressed levels of air charge to an intake manifold.
As a multi-stage system example, exhaust gas may flow through a primary turbine at low engine speeds (e.g., less than 3,000 rpm). The flow of the exhaust gas can increase boost pressure in the primary turbine. The boost pressure increases as the engine speed increases. The boost pressure may be controlled by opening a bypass valve (BPV). For example, the BPV may be opened to allow the exhaust gas to bypass the primary turbine when the boost pressure is greater than a predetermined threshold.
The compressed exhaust gas from the primary turbine may be directed to a secondary turbine. The secondary turbine is operated at greater boost pressure than the primary turbine at high engine speeds (e.g., greater than 3,000 rpm). The boost pressure in the secondary turbine may be controlled by opening a wastegate. For example, the wastegate may be opened to allow the exhaust gas to bypass the secondary turbine to reduce pressure.
Reducing boost pressure via the BPV and/or the wastegate can cause a pressure drop in an intake manifold. An engine control system may compensate for the pressure drop by maintaining a throttle position or by increasing a throttle opening to increase air flow into cylinders of an engine. This compensation can cause an increase in vehicle acceleration. The increase in vehicle acceleration is referred to as a sail-on condition. The sail-on condition can increase exhaust emissions; cause engine instability; and/or reduce fuel economy.